Application of Electron Energy Loss Spectroscopy and Energy Filtering Transmission Electron Microscopy for Microchemical Studies in 2.25Cr-1Mo Steel

Electron enregy loss spectroscopy (EELS) and energy filtering transmission electron microscopy (EFTEM) investigation on 2.25Cr-lMo steel was carried out to understand the nature of evolution of secondary carbides. The filtered images obtained from two different ageing treatments indicate that the steel evolves to a more stable carbide namely M23C6 in comparison to M2C. Microchemical information was generated from EELS spectra. Suitable choice for estimating the microchemical state was discussed. To evaluate the behaviour of ageing an elemental ratio of Fe to Cr is employed.     

2.25Cr-1Mo钢后续热处理中的磷偏聚行为

根据溶质晶界偏聚理论和原子扩散原理研究了2.25Cr-1Mo钢服役12年后磷晶界偏聚行为及脱脆处理工艺。采用俄歇电子能谱实验对溶质原子的晶界偏聚程度进行了测试分析。结果表明:对于已脆化的2.25Cr-1Mo钢,存在临界脆化温度,且脆化程度越高,临界脆化温度越低;高于临界脆化温度数小时的保温即可消除脆化现象,即2.25Cr-1Mo钢的脱脆速率远大于脆化速率。     

Thermal diffusivity measurement of 2.25Cr-1Mo steel with internal friction

In this work, the internal friction of 2.25Cr-1Mo steel measured on a dynamic mechanical analyzer (DMA) was used to evaluate its thermal diffusivity at elevated temperatures. With the dual cantilever clamp, the internal friction of 2.25Cr-1Mo steel at high temperatures was obtained, which was mainly composed of the thermoelastic relaxation with a peak of 15 Hz and the broad spectrum of relaxation processes especially at low frequencies. According to the deconvolution of these results, the thermoelastic internal friction of 2.25Cr-1Mo steel at high temperatures was achieved. Then, the thermal diffusivity of 2.25Cr-1Mo steel at high temperatures was quantitatively estimated, which confirmed the validity of present method.     

Damage mechanism in weldment of 2.25Cr-1Mo steel under creep-fatigue loading

The purpose of this study is to clarify damage process of “Type IV cracking” in weldment of a 2.25Cr-1Mo steel and to propose a micro damage prediction method. From continuous observation under a creep-fatigue test, it was found that spherical shape voids initiated and grew on grain boundaries in fine grain region and these voids grow continuously by changing their shape to crack-like. Both spherical and crack-like void growth rate equations were derived from the proposed void growth model. It was indicated that measured void growth rate under the creep-fatigue loading was well predicted by the growth rate equations.     

The effect of stress relaxation loading cycles on the creep behaviour of 2.25Cr-1Mo pressure vessel steel

This paper investigates the effects of repeated stress relaxation loadings and post stress relaxation creep to assess the stress relaxation-creep interaction and microstructural evolution of 2.25Cr-1Mo steel. Prior to creep testing, the microstructure of the material subjected to stress relaxation exhibited a structure which was non-conservative in predicting the remaining creep life of the material. The results obtained in the test program showed that the damage due to the effects of stress relaxation was crucial and had a significant effect on the creep life of this material. The study has also shown that the extent of metallurgical degradation, due to stress relaxation, may not be evident through microstructural assessment. Consequently, established life assessment procedures may not represent conservative estimates of remaining life because the microstructural indicators of damage, due to stress relaxation, are not evident.The effects of stress relaxation on the creep properties of ferritic pressure vessel steel are life limiting for critical high temperature power generation plant. In this study a comprehensive test program has been undertaken to assess the interaction of stress relaxation with creep and microstructural evolution in 2.25Cr-1Mo steel.     

2.25Cr-1Mo-O.25V低合金耐热钢中贝氏体的形成机制研究

采用真空冶炼炉和可逆式轧机制备出试验材料,将其在940℃奥氏体化后分别水淬到550℃、650℃和750℃,再空冷至室温,然后采用扫描电镜和透射电镜观察材料的显微组织,发现分别主要为下贝氏体、上贝氏体和粒状贝氏体.上贝氏体组织和下贝氏体组织均以扩散机制形成,而区域冷却速率的差异是造成粒状贝氏体组织基体形貌差异的主要原因.     

Creep-fatigue life evaluation for weldments of modified 9Cr-1Mo steel at boiler temperatures

Creep-fatigue tests were conducted to verify long-term structural integrity for weldments of modified 9Cr-1Mo steel at boiler plants. As a result, it was proved that the weldments were fractured mainly at the HAZ and the fatigue lives were reduced by strain hold. This appeared to be due to acceleration of crack initiation by creep strain concentration and acceleration of crack growth by void generation in the HAZ. A method for predicting the life is proposed based on the fracture mechanism, and its accuracy was confirmed to be within a factor of two.     

Optimization of Welding Process Parameters for 9Cr-1Mo Steel Using RSM and GA

Optimization of A-TIG welding process parameters for 9Cr-1Mo steel has been carried out using response surface methodology (RSM) and genetic algorithm (GA). RSM has been used to obtain the design matrix for generating data on the influence of process parameters on the response variables. A second-order response surface model was developed for predicting the response for the set of given input variables. Then, numerical and graphical optimization was performed using RSM to obtain the target depth of penetration (DOP) and heat-affected zone (HAZ) width using desirability approach. Multiple regression models were developed based on the generated data, and then the models were used in GA to determine the optimum process parameters for achieving the target DOP and HAZ width. GA-based models employed two different selection processes. Both the RSM- and GA-based models suggested a number of solutions in terms of process parameters, and the identified solutions were validated by experiments. GA-based model employing tournament selection has been found to be a more accurate method for determining the optimum A-TIG welding process parameters.     

Characterization of Hydrogen Embrittlement in 2.25cr-1mo-0.25v Steel by Eddy Current Method

Eddy current method has been recently developed to characterize mechanical properties of materials and assess internal hydrogen content of high strength low alloy steels. The application of eddy current testing in evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V ferritic steel has been investigated using tensile test and electrochemical hydrogen charging test. The relationship between the embrittlement index and eddy current signal is well established. It found that there is a good linear relation between the hydrogen-induced plasticity loss and the eddy current signal. This shows eddy current testing is an effective method for evaluating hydrogen embrittlement state of 2.25Cr-1Mo-0.25V steel.     

Hollow Ptychography: Toward Simultaneous 4D Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy

With the recent development of high-acquisition-speed pixelated detectors, 4D scanning transmission electron microscopy (4D-STEM) is becoming routinely available in high-resolution electron microscopy. 4D-STEM acts as a “universal” method that provides local information on materials that is challenging to extract from bulk techniques. It extends conventional STEM imaging to include super-resolution techniques and to provide quantitative phase-based information, such as differential phase contrast, ptychography, or Bloch wave phase retrieval. However, an important missing factor is the chemical and bonding information provided by electron energy loss spectroscopy (EELS). 4D-STEM and EELS cannot currently be acquired simultaneously due to the overlapping geometry of the detectors. Here, the feasibility of modifying the detector geometry to overcome this challenge for bulk specimens is demonstrated, and the use of a partial or defective detector for ptycholgaphic structural imaging is explored. Results show that structural information beyond the diffraction-limit and chemical information from the material can be extracted together, resulting in simultaneous multi-modal measurements, adding the additional dimensions of spectral information to 4D datasets.     

The Ductile to Brittle Transition Behavior of the Modified 9Cr-1Mo Steel and Its Laser Welds

The ductile to brittle transition temperature (DBTT) of the modified 9Cr-1Mo steel and its laser welds was studied. The increase in grain size of the weld structure ascended the DBTT of the steel significantly. The transformation of retained austenite at martensite interlath boundaries into untempered and/or twinned martensite could also contribute to increased DBTTs of the steel and its welds tempered at 540℃.     

The Ductile to Brittle Transition Behavior of the Modified 9Cr-1Mo Steel and Its Laser Welds

The ductile to brittle transition temperature (DBTT) of the modified 9Cr-lMo steel and its laser welds was studied. The increase in grain size of the weld structure ascended the DBTT of the steel significantly. The transformation of retained austenite at martensite interlath boundaries into untempered and/or twinned martensite could also contribute to increased DBTTs of the steel and its welds tempered at 540℃.     

Testing of welded 2.25CrMo steel, in hot, high-pressure hydrogen under creep conditions

The standard 2.25Cr-1Mo steel, has been used for most of the hydroprocessing reactors built during the 30 years up to the end of the 1990’s. Under service conditions, the steel, and in particular its weldments, may experience degradation by “hydrogen attack”, or high-temperature hydrogen attack (HTHA), which is responsible for several failures and even casualties occurring in the history of the industry. The selection of the structural steels is still based on the empirical Nelson curves which, among other limitations, do not provide information on the behaviour of weldments. This causes uncertainties in the definition of the safety boundaries for operation of pressurized components in the petrochemical industry. The results of the present study confirm that weldments are a weak point of the pressurized components and suggest that the HAZ, and in particular the ICHAZ, is the most sensitive metallurgical zone to hydrogen attack.     

An improvement in creep strength of thermo-mechanical treated modified 9Cr-1Mo steel weld joint

Modified 9Cr-1Mo steel weld joints generally experience the type IV premature failure in the intercritical region (ICR) of HAZ under long term creep exposure at high temperature. Possibility of improving the resistance of this joint to type IV cracking through thermo-mechanical treatment (TMT) of the steel has been explored. Weld joints have been fabricated from the TMT and conventional normalized and tempered (NT) steels using electron beam (EB) welding process. Creep tests have been carried out on NT and TMT steels joint at 923 K (650°C) and 110–100 MPa applied stress. Creep rupture life of the TMT weld joint was significantly higher than the NT steel weld joint. Significant variations of microstructural constituents such as M₂₃C₆ precipitate; lath structure and hardness across the joint have been examined in both the joints. The coarser M₂₃C₆ precipitate and lath, and subgrain formation in the ICR resulted in the soft zone formation and was predominant in the ICR of NT steel joint. The enhanced MX precipitation through TMT processing and reduction in coarsening of M₂₃C₆ precipitate under thermal cycle resulted in improved creep rupture strength of TMT steel weld joint.     

金红石对1Cr-0.5Mo合金系焊条工艺性能和熔敷金属力学性能的影响

研究了金红石对1Cr-0.5Mo合金系焊条工艺性能和熔敷金属力学性能的影响。结果表明,金红石对该合金系焊条的工艺性能和熔敷金属力学性能均有较大的影响。随着药皮中金红石加入量的增加,焊条工艺性能得到改善,熔敷金属的强度增加,-20℃夏比冲击功降低。     

Fatigue life estimation of notched specimens of modified 9Cr-1Mo steel under uniaxial cyclic loading

Accuracy in the estimation of low cycle fatigue life of modified 9Cr-1Mo steel notched specimen by different analytical methods such as linear rule, Neuber’s rule, strain energy density method and numerical method such as finite element analysis have been studied in this investigation. The fatigue tests on notched specimens having notch radius of 1.25 mm, 2.5 mm and 5.0 mm were carried out at 823 K with net stress amplitudes of 250 MPa, 300 MPa and 350 MPa. The fatigue tests on smooth specimens were carried out with strain amplitudes ranging from ±0.3% to ±0.8% with a strain rate of 3 × 10^?3 s^?1 at 823 K to evaluate the fatigue life of notched specimen through strain-life approach. In order to predict the cyclic stress response of the material, Chaboche non-linear hardening model was employed considering two back stress components. Predicted hysteresis loops for smooth specimen were well in agreement with experimental results. Estimated fatigue lives of notched specimens by analytical methods and finite element analysis were within a factor ±16 and ±2.5 of the experimental lives respectively.     

An FEGSTEM Study of Grain Boundary Segregation of Phosphorus during Quenching in a 2.25Cr-1Mo Steel

Quenching-induced phosphorus segregation to prior austenite grain boundaries in a 0.077 wt pct P-doped 2.25Cr-1Mo steel is examined using field emission gun scanning transmission electron microscopy (FEGSTEM). A phosphorus level of around 1.56 at. pct is observed for the water-quenched sample. In recognition of insufficiently high spatial resolution of the technique for grain boundary composition analysis, the measured results are corrected by an analytical convolution method. The corrected phosphorus segregation level may be up to about 4.7 at. pct. The quenchinginduced phosphorus segregation is nonequilibrium segregation and the migration of vacancy-phosphorus complexes plays an important role in the kinetic process. For such a reason, the mechanism for migration of the complexes is discussed in some detail.     

Influence of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint

Effect of joint thickness on Type IV cracking behaviour of modified 9Cr-1Mo steel weld joint has been investigated. Creep tests on multi-pass double-V cross weld joint flat specimens of the steel having thicknesses in the range 1–17 mm have been carried out at 923 K (650°C) and 50 MPa stress. Creep rupture life of the weld joint was found to increase with thickness and reached a maximum value around 10 mm of thickness followed by decrease with further increase in thickness. Failure in the weld joints occurred in the soft intercritical region of the heat-affected zone (HAZ). Creep strain localisation was observed at the fractured location and was more in the thinner weld joints than in the thicker weld joint. Creep cavitation in the intercritical region of HAZ close to the unaffected base metal was more extensive at the mid-location of the weld pass, where the HAZ width was relatively larger and hardness was lowest. The type IV cavitation in intercritical HAZ was more extensive in thicker joint, whereas creep strain concentration in the intercritical HAZ was more in thinner weld joint. Creep cavitation in the joint was more pronounced at near mid-thickness locations than those beneath the specimen surface. Joints of intermediate thickness possessed higher creep rupture life because of relatively less accumulation of creep deformation coupled with lower creep cavitation in the intercritical region of HAZ.     

An investigation on oxidation/carburisation of 9Cr-1Mo steel heat exchanger tube in an AGR environment

Abstract

9Cr–1Mo steels have been used extensively in the power generation industry. In this study, a wide range of experimental samples exposed at different times and temperatures in a CO₂ environment were analysed to look at the development of the metal and oxides over time. The main objective of this work was to obtain a better understanding of the carburisation and oxidation behaviour of 9Cr 1Mo steels as a function of temperature/time, with special attention paid to the transition from protective to breakaway oxidation. In addition, experiments were also carried out to investigate any links between oxidation transition and carburisation behaviour of these materials.     

几种结构钢在人造海水中自腐蚀电位和析氢电位的研究

本文介绍了16Mn钢和2 1/4Cr-1Mo钢[注1]在人造海水中的自腐蚀电位和析氢电位的测量原理和方法。在实验中用怛电位仪测量不同试样的自腐蚀电位和析氢电位;研究了温度对试样自腐蚀电位的影响;不同热处理方法对试样自腐蚀电位和析氢电位的影响。